This invention relates to a pressure sensitive copying system and more particularly to pressure sensitive copying paper having a good light resistance, a good color formability and an improved printability.
Usually the so-called "pressure sensitive copying system" consists of these three kinds of basic sheets such as top sheet, middle sheet and bottom sheet, wherein the top sheet is coated on the underside thereof with a composition consisting mainly of pressure-rupturable microcapsules each enclosing an oil droplet containing an electron donating organic chromogenic material (hereinafter referred to as "color former") dissolved or dispersed therein, the middle sheet is coated on the upperside thereof with another composition consisting mainly of electron accepting acidic reactant material (hereinafter referred to as "acceptor") which will produce a colored image when contact with the color former and also is coated on the underside thereof with the composition of microcapsules containing oil droplets in which a color former is dissolved or dispersed and the bottom sheet is coated on the upperside thereof with the composition of acceptor. One top sheet and one bottom sheet or, one top sheet, at least one middle sheet and one bottom sheet are superposed in that order to form a set of copying sheet in such a manner that the microcapsule coating layer and the acceptor coating layer are in contact with each other in each adjoining two sheets. Any partial pressing on the upperside of the top sheet of the thus prepared copying system with a pen or a typewriter will break the microcapsules positioned on the pressing, resulting in making the color former react with the acceptor so as to develop a color only on the part pressed.
In another pressure sensitive copying system, there are disposed on one surface of the same sheet both the acceptor and the microcapsules containing oil droplets in which the color former is dissolved or dispersed. This system is known as the "self contained" system.
Recently, various organic acid materials have found their usefulness as the acceptor for the pressure sensitive copying system. Among those organic acceptors there ae included phenols, phenolic polymers, carboxylic acids and other acidic polymers.
In comparison with inorganic acceptors such as acid clay and activated clay, the above mentioned organic acceptors are advantageous and have a good reputation for the reason that they have a good color formability which can be retained for a long time and the color images produced are stable and have a good light resistance and a good moisture resistance. However, the color images produced on such organic acceptors never become stable until several to twenty hours or more after the color images has been developed and the light resistance of the color images immediately after the color developing is rather unsatisfactory.
The principal object of the invention is to provide an improved pressure sensitive copying system in which the light resistance of the developed color images, especially the light resistance immediately after the color developing, is remarkably enhanced.
Another object of the invention is to provide an improved pressure sensitive copying sheet or paper having a good color formability.
A further object of the invention is to provide an improved pressure sensitive copying sheet or paper having a good printability.
Other objects and advantages of the invention will be apparent from the following description.